1. Field of the Invention
This invention relates to systems for mixing a plurality of video images to form a single composite image and more particularly to a spatial transformation system providing a key signal which precisely defines a desired image viewing region notwithstanding a spatial transformation of the image.
2. Discussion of the Prior Art
It is a common practice within the video broadcast industry to selectively mix two or more video images to generate a single combined image. A typical example occurs where a TV newscaster is shown describing a news event while a video image of the news event is shown as an insert located slightly to one side of the announcer. Typically this is accomplished by presenting both the primary image showing the announcer and the secondary image containing the insert to a mixer or switches. A one bit resolution or on/off key signal is also presented to the mixer. When the key signal is off the primary image is output by the mixer and when the key signal is on the secondary or insert image is output by the mixer to provide a combined video image at the output. The key signal may be either a high or a low monochrome intensity level or in some systems may be a chroma key representing a precisely selected color value. A chroma key processing system is described in U.S. Pat. No. 4,240,104 entitled "Measurement of Chroma Key Area in Television Systems".
Known key generating systems provide a rectangular viewing area in the 4:3 aspect ratio of a standard television set and provide operator control over the size and location of the keyed image region. The camera for the secondary image must then be carefully aligned so that the portion of the image which is to be displaced to the viewing audience is located within this key selected viewing region.
The problem of aligning the camera with the key selected viewing region is reduced somewhat by spatial transformation systems which are also known as special effect systems. These systems permit the camera to center upon the desired image activity in a normal manner with the image then being transformed in size and location under operator control to match the key selected viewing region. However, constraints imposed by the rectangular shape of the key selected viewing region severely limited use of functions of these transformation systems such as perspective rotation which result in an irregularly shaped output video image. Furthermore, problems occur in precisely aligning the secondary image with the key signal defined viewing region to eliminate unsightly boundaries and aliasing between the primary and secondary images.